The present invention relates to an arrangement for supplying air in an air jet form to the furnace of a recovery boiler. The furnace has a front wall, a rear wall and side walls. Black liquor spraying devices are disposed on these walls at one or several levels. A plurality of air ports are located on several horizontal levels on said walls for introducing air into the furnace from an air supply. Specifically, the invention relates to an arrangement for organizing the secondary air flows below the black liquor spraying devices.
An optimal supply of combustion air in the lower part of the furnace of a black liquor recovery boiler plays a considerable role in the control of a combustion process in the boiler.
Since the chemical reactions in the kraft recovery boiler are very rapid, the speed of the process becomes substantially dependent on the mixing of combustion air and black liquor. This mixing step determines the burning rate and also has an effect on the process efficiency. Air and black liquor are typically introduced into the boiler through individual ports, and it is particularly important that a rapid mixing in the boiler is effected by the air supply without generating large differences in the upward flow profile. The high velocity xe2x80x9cliftxe2x80x9d in the center of the furnace is especially harmful as it results in carry-over of the sprayed liquor droplets. The burning symmetry must be controlled throughout the whole cross-sectional area of the boiler and the air supply must be adjusted when required.
Black liquor is generally introduced in the form of considerably large droplets into a kraft recovery boiler so as to facilitate the downward flow of the droplets, and to prevent them from flowing, unreacted (as fine fume), upwards together with the upward flowing gases to the upper part of the boiler. The large droplet size, which results in the droplets being spaced further from each other than in a fine black liquor spray, means that proper mixing is even more important in a recovery boiler. Pyrolysis of black liquor solids produces char as well as combustible gases. The char falls down to the bottom of the furnace and forms a char bed which must be burned.
A stoichiometric amount of air, relative to the amount of black liquor, is introduced into the recovery boiler and additionally, a surplus amount of air is supplied to ensure complete combustion. Too much excessive air, however, causes a loss in efficiency of the boiler and an increase in costs. Air is usually introduced into the boiler on three different levels: primary air at the lower part of the furnace, secondary air above the primary air level but below the liquor nozzles, and tertiary air above the liquor nozzles to ensure complete combustion. Air is usually introduced through several air ports located on all four furnace walls, or only on two opposing walls of the furnace.
Primary air is typically 20-35% of the total air supplied into the furnace, depending on liquor and dry solids content of the liquor. The task of the primary air is to keep the char bed from rising into air ports of the furnace. Secondary air is typically 35-60% of total air, and tertiary air, which may be distributed into several levels in vertical direction, is typically 10-40% of the total air. More than three air levels for introducing air into the furnace may be arranged in the boiler.
Mixing of black liquor and air is difficult because of the upflow of gas which is formed in the center part of the boiler, through which it is difficult for the weak secondary air flow to penetrate. More specifically, the primary air flows, supplied from the sides in the bottom part of the boiler, collide with each other in the center part of the boiler and form, with secondary air flow pattern, in the center part of the boiler, a gas flow flowing very rapidly upwards, catching flue gases and other incompletely burnt gaseous or dusty material from the lower part of the furnace. This gas flow, also called a xe2x80x9cdroplet liftxe2x80x9d, also catches black liquor particles flowing counter-currently downwards and carries them to the upper part of the boiler, where they stick to the heat surfaces of the boiler, thus causing fouling and clogging. In the center part of the boiler, the speed of the upwards flowing gas may become as much as four times as great as the average speed of the gases as a result of incomplete or weak mixing. Thus, a zone of rapid flow is formed in the center part of the boiler, and this renders mixing of flue gases from the side of the flow very difficult to achieve.
The xe2x80x9cdroplet liftxe2x80x9d mentioned above, results in such a situation where the tertiary air(s) has (have) to burn not only the unburned gases from combustion (CO, H2S, NH3, etc.), but the unburned char from the droplets as well. As the combustion rate for char is much slower than for the unburned gases, increased excess oxygen has to be used to ensure complete combustion. Then the flue gas leaving the furnace contains higher amounts of residual CO and H2S, and the utilization of the furnace is less effective than would be possible.
Current secondary air arrangements are also characterized by at least one secondary air level where secondary air ports are placed close to another in horizontal direction. This leads to mixing patterns where furnace gases are circulated in vertical direction, with the above mentioned xe2x80x9cliftxe2x80x9d, i.e. they flow towards the walls and then turn up (or down) and follow the main flue gas direction.
Another variation of the secondary air design is to use partial interlaced jets (e.g. U.S. Pat. Nos. 5,121,700, 5,305,698), whereby a large jet opposes a small jet. The large and small jets are alternated between the two opposite walls used.
U.S. Pat. No. 5,724,895 discloses an arrangement for feeding combustion air. In this system, a more favorable flow pattern in furnaces can be achieved by replacing vertical mixing by horizontal mixing, whereby a strong central flow channel, upward xe2x80x9cliftxe2x80x9d, can be prevented. This horizontal mixing is applied for the whole furnace. The horizontal mixing is improved by disposing additional air inlet ports e.g., at more than six different elevations in a pattern of vertical spaced-apart rows above the lowest air levels.
In the method of U.S. Pat. No. 5,454,908 a portion of combustion air is introduced into a recovery boiler at a distance above the black liquor inlet so as to provide a reducing atmosphere with a residence time of at least three seconds between the black liquor inlet and the introduction of said portion of combustion air. A drawback of the described arrangement is a high vertical combustion area, reaching in extreme cases the bullnose of the furnace. As this combustion area has a reducing atmosphere, at least locally, more expensive materials have to be used in the furnace to a higher position than would be needed if combustion took place lower in the furnace. Other disadvantages of the air systems, where combustion takes place high up in the furnace include high furnace outlet temperature resulting in large convective heat transfer surfaces later in the boiler, lower temperature in the lower furnace, and more expensive layout. The lower temperature in the lower furnace does not allow as high sulfidity without SO2 emissions as a combustion system having a higher lower furnace temperature does.
The present invention provides an improved air supply system of combustion air to a recovery boiler. Particularly, a secondary combustion air supply is provided in which either local and/or central upward gas flows having a high velocity compared to an average upward gas velocity are efficiently avoided. Another feature of the invention is to enable a constant penetration of combustion air into the boiler at different loading levels. A further feature of the invention is to produce a better mixing of black liquor and combustion air in the furnace. The improved air supply arrangement of this invention is also designed to reduce the amount of harmful emissions from the boiler furnace.
The present invention may be embodied in a recovery boiler having a furnace that comprises:
two horizontal air levels at different elevations are arranged above the lowest air level or levels and below the black liquor spraying level or levels,
air is supplied from two opposite walls on said two levels and the air ports on each level are located so that the air jets are introduced in an interlaced pattern having an even number of jets on one opposite wall and an uneven number of jets on the other opposite wall, and
the air jets of said two air levels are located in substantially vertical rows.
According to an embodiment of the invention, secondary air on two air levels is introduced only from the two opposite walls, preferably from the front and rear walls. Substantially no air is supplied from the two remaining walls, i.e., the side walls. In the interlaced pattern, an air flow coming from an air port located on a wall having an even number of air jets is directed in between two adjacent air ports of the opposite wall having an uneven number of air jets. Correspondingly, the air jets coming from the wall having an uneven number of air jets are directed substantially directly in a horizontal plane towards the opposite wall. The air jets coming from the opposite walls by-pass each other without actually colliding with each other.
Thus, on the two secondary levels, the lateral arrangement of the jets on one level sideways is symmetrical. On the wall having an uneven number of air jets, e.g. three, the middle air jet is located substantially on the center line of the wall, and the other jets are located within an equal distance on both sides of the middle jet. On the opposite wall having an even number of jets, two in this example, the jets are located laterally midway between the jets on the opposite wall. Thus, the jet arrangement is symmetrical in relation to the vertical plane parallel to the remaining walls (i.e. the walls having no secondary air jets) and passing through the center lines of the walls having the secondary air jets.
The present invention employs the following principles in order to avoid strong vertical gas flows, but still to obtain effective mixing in the furnace between combustion air and unburned/burning liquor droplets:
strong secondary air jets (strong air jets below black liquor spraying devices).
arrange these jets so that they do not collide against each other, which easily generates strong upflow jets and unwanted upflow profile for the gases in the furnaces. Instead, strong shearing flows should be generated to obtain good mixing.
minimize suction of gases in vertical direction into these jets above the liquor spraying devices as this increases gas flow up.
minimize suction of liquor droplets from liquor sprays into tertiary air jets.
cover the tertiary air stage(s) with several jets, which cover the furnace cross section evenly and well in order to prevent the formation of vertical jets that may punch the final combustion area where the final combustion of the unburned gases cannot take place. Also, here the jets should not collide against each other but generate strong shearing flows and good mixing.
According to a preferred embodiment of the invention, there is a distance, V, in vertical direction between the horizontal air levels, when measured from the lateral center lines of the air ports of the air levels. This distance, V, fulfills the following formula: V/Lxe2x89xa60.5, where L is the distance between two adjacent air ports on the same air level, when measured from the longitudinal center lines of the adjacent air ports. Preferably, V/L is 0.25-0.5. Typically, the vertical distance, V, is 1-2 meters.
Preferably the air ports located one above the other are positioned in a vertical row so that they are located on the same straight vertical line. The invention covers also an embodiment in which the air ports laterally deviate so that there a transverse distance, D, between the air ports above each other. The transverse distance is a distance between the longitudinal center lines of the ports one above the other. D is less than 1.5xc3x97H or less than 1.5xc3x97W depending on which number is greater. H is the height of the highest air port and W is the width of the widest air port.
According to an embodiment of the invention there is only one air level below the two secondary air levels. According to another embodiment, the number of the lowest air levels below the two secondary air levels is two. The air jets of the air level which is located higher in vertical direction below the two secondary air levels are arranged in an interlaced pattern on two opposite walls, preferably on the front and rear walls, so that the number of air jets is greater by one than the number of air jets of the two secondary air levels on the same wall. For example, if the secondary air level has one air jet on the front wall and two jets on the rear wall, the above-mentioned lower air level has two air jets on the front wall and three jets on the rear wall. However, the air velocity is lower on this lower air level. On this air level which, thus, is located above the lowest air level and below the two secondary air levels, and which can be called a low-secondary or high-primary air level, the air jets are arranged also on the remaining opposite walls, i.e., preferably on the side walls. The air jets on the side walls are smaller than the air jets on the front and rear walls.
An embodiment of the invention is directed also to an arrangement for supplying secondary air in an air jet form to the furnace of a recovery boiler, said furnace having a front wall, a rear wall and side walls, black liquor spraying devices disposed on said walls on a level and a plurality of air ports located on several horizontal levels on said walls for introducing air into the furnace from an air supply, said arrangement comprising: one horizontal air level is arranged above the lowest air level or levels and below the black liquor spraying level or levels, and air is supplied from two opposite walls on said level and the air ports are located so that the air jets are introduced in an interlaced pattern having an even number of jets on one opposite wall and an uneven number of jets on the other opposite wall. The air port has an area, A, and a width, W, and the ratio of the area to the width to the power of 2 (A/W2) is more than 4, preferably more than 10.